There have recently been proposed nonvolatile semiconductor storages, such as a flash memory including a memory cell with a select transistor and a memory transistor. As memory transistors, a floating gate type memory transistor and a SONOS (Silicon Oxide Nitride Oxide Silicon) type memory transistor are known.
In a SONOS type memory transistor, an insulating film having a structure, in which a lower insulating film, a charge storage film, and an upper insulating film are stacked, is used as a gate insulating film. In the SONOS type memory transistor, data storage or erasure is performed by loading or unloading charges into or from the charge storage film.
[Patent document] Japanese Laid-open Patent Publication No. 2010-258250